Based on the ability of various materials to conduct current, the
materials are classified as conductors, insulators and the
semiconductors.

A metal which is very good carrier of electricity is called conductor.
A very poor conductor of electricity is termed as insulator. A metal
having conductivity which is between conductor and an insulator is
called semiconductor. Copper and aluminium are good examples of a
conductor. Glass, wood, mica, diamond the examples of an insulator which
do not conduct current. Silicon and germanium are the examples of a
semiconductor which do not conduct current at low temperatures but as
temperature increases these materials behave as good conductors. Let us
see the energy band diagrams for these three types of metals.

1.1 Conductors

It has been mentioned earlier that a material having large number of
free electrons can conduct very easily. For example, copper has 8.5 x
1028 free electrons per cubic metre which is a very large
number. Hence copper is called good conductor. In fact, in metals like
copper, aluminium there is no forbidden gap between valence band and
conduction band. The two band overlap. Hence even at room temperature, a
large number of electrons are available for conduction. So without any
additional energy, such metals contain a large number of free
electrons and hence called good conductors. An energy band diagram for a
conductor is shown in the Fig. 1(a).

1.2 Insulators

An insulator has an energy band diagram as shown in the Fig. 1(b). In
case of such insulating material, there exists a large forbidden gap in
between the conduction band and the valence band. Practically it is
impossible for an electron to jump from the valence band to the
conduction band. Hence such materials can not conduct and called
insulators. The forbidden gap is very wide, approximately of about 7 eV
is present in insulators. For a diamond, which is an insulator, the
forbidden gap is about 6 eV. Such materials may conduct only at very
high temperatures or if they are subjected to high voltage. Such a
conduction is rare and is called breakdown of an insulator. The other
insulating materials are glass, wood, mica, paper etc.

Fig. 1 Energy band diagrams

1.3 Semiconductors

Now let us come to an important category of materials, which are
neither insulators nor conductors. The forbidden gap in such materials
is very narrow as shown in Fig. 1(c). Such materials are called
semiconductors. The forbidden gap is about 1 eV. In such materials, the
energy provided by the heat at room temperature is sufficient to lift
the electrons from the valence band to the conduction band. Therefore at
room temperature, semiconductors are capable of conduction. But a 0o K or absolute zero (-273o C), all the electrons of semiconductor materials find themselves locked in the valence band. Hence at 0o
K, the semiconductor materials bahave as perfect insulators. In case of
semiconductors, forbidden gap energy depends on the temperature. For
silicon and germanium, this energy is given by,

where T = Absolute temperature in K

Assuming room temperature ti be 27o C i.e. 300o
K, the forbidden gap energy for Si and Ge can be calculated from the
above equations. The forbidden gap for the germanium is 0.72 eV while
for the silicon it is 1.12 eV at room temperature. The silicon and
germanium are the two widely used semiconductor materials in electrons
devices, as mentioned earlier.

key Point : While calculating , substitute T in ok.Why silicon is most widely used ?

Looking at the structure of silicon and germanium atom, it can be seen that valence shell of silicon is 3rd shell while valence shell of germanium is 4rd
shell. Hence valence electrons of germanium are at larger distance from
nucleus than valence electrons of silicon. Hence valence electrons of
germanium are more loosely bound to the nucleus than those of silicon.
Thus valence electrons of germanium can easily escape from the atom, due
to very small additional energy imparted to them. So at high
temperature, germanium becomes unstable than silicon and hence silicon
is widely used semiconductor material.

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